Various techniques have been developed within the telecommunications industry for transmitting voice and data signals to residences and businesses over existing copper telephone lines. A telecommunication service which utilizes an existing copper infrastructure is often referred to as a Plain Old Telephone Service (POTS). A telecommunications system that provides for the transmission of mixed voice/data signals utilizes a device that separates mixed voice/data signal into a lower frequency voice signal and a higher frequency data signal. Such devices are commonly referred to as POTS splitters.
An ADSL (Asymmetrical Digital Subscriber Line) service, for example, provides for the concurrent transmission of voice and data signals over conventional copper connections. A significant advantage of an ADSL service concerns the capability to provide both voice and data to a home or business using a single telephone connection. Such ADSL services require a POTS splitter to extract the voice and data signals from the mixed voice/data signal transmitted over the single telephone connection.
A common POTS splitter design employs both a lowpass filter and a highpass filter. The lowpass filter is used to extract the voice signal from the mixed voice/data signal. The highpass filter is used to extract the data signal from the mixed voice/data signal. In telecommunications systems that employ a billing tone or metering tone, the POTS splitter design must additionally provide filtering circuitry to extract the billing/metering tone. Such billing/metering tones, which generally have a frequency between the voice signal frequency band and the data signal frequency band, are commonly used in European telecommunications systems.
A typical POTS splitter filter design which accommodates billing/metering tones includes an order-n low pass filter with a corner frequency located at some point above the billing/metering tone frequency. While easy to design and implement, a significant disadvantage associated with traditional POTS splitter filter designs concerns audible xe2x80x9chissingxe2x80x9d or other undesirable signal components which are passed through to the voiceband terminating device.
Unwanted spectral elements associated with frequencies between the voiceband and the databand may or may not be detectable by the human ear. However, such unwanted spectral elements pose a definite problem for data terminating devices, such as modems, fax machines, and the like. Problems associated with undesirable signal components associated with frequencies between the voiceband and the billing/metering tone frequency, for example, include degraded bit-error rate (BER) performance, lost data, and dropped connections. Such problems may adversely affect both the customer premise equipment (CPE) and the Central Office (CO) terminating equipment.
There exists a need in the telecommunications industry for an improved POTS splitter device that provides for filtering of voice, data, and billing/metering tone content of a mixed or composite voice/data signal. There exists a further need for a POTS splitter filter that effectively passes voice and billing/metering tone content while suppressing undesirable signal components associated with frequencies between the voiceband and the billing/metering tone frequency. The present invention fulfills these and other needs.
The present invention is directed to an apparatus and method for filtering voiceband and specified tone frequencies of a mixed voice/data signal. A specified tone frequency may be representative of a billing or metering tone. The mixed voice/data signal may conform to an Asymmetrical Digital Subscriber Line (ADSL) standard.
An apparatus according to an embodiment of the present invention includes a lowpass filter section that passes voiceband content of the mixed signal. The lowpass filter section suppresses mixed signal content having frequencies falling within a databand of the mixed signal. The apparatus further includes a bandpass filter section coupled with the lowpass filter section. The bandpass filter section passes the metering tone frequency.
The lowpass and bandpass filter sections further exhibit a combined or composite frequency response that provides for a stopband between the voiceband and the metering tone frequency. The bandpass filter section effectively detunes a frequency response of the lowpass filter section to provide for the stopband between the upper end of the voiceband and the metering tone frequency. The stopband suppresses undesirable mixed signal content having frequencies falling within the stopband. The stopband associated with the lowpass and bandpass filter sections preferably provides for an insertion loss of 3 dB or greater between the voiceband and the metering tone frequency.
The lowpass filter section and the bandpass filter section may each be implemented to include only passive components. For example, the lowpass and bandpass filter sections may be implemented to include passive magnetic components. The lowpass and bandpass filter sections, for example, may include a number of filter stages, wherein each stage includes a pair of coupled inductive elements and a load capacitor. The pair of coupled inductive elements and the load capacitor of at least one of the stages preferably has respective inductance and capacitance values differing from those of other stages.
In one embodiment, the bandpass filter section is integral with the lowpass filter section. According to this embodiment, the integral bandpass filter section is active upon detuning of the lowpass filter section. The passband of the bandpass filter is defined by the upper end of the stopband and the cutoff frequency of the lowpass filter section. The bandpass filter section may define a resonant tuned bandpass filter centered around the metering tone frequency. In an alternative embodiment, the lowpass and bandpass filter sections are separate filter elements coupled together.
The metering or other type of specified tone typically has a frequency specified by a telecommunications standard. The metering tone, for example, may have a frequency of about 12 kHz, 16 kHz or 20 kHz. The voiceband content of the mixed signal generally includes frequencies to about 4 or 5 kHz. The databand is generally defined to include mixed signal content having frequencies of about 30 kHz or higher.
The filtering apparatus may be configured as a 3-port filter. In this configuration, the lowpass and bandpass filter sections define two ports of the 3-port filter. A highpass filter section defines a third port of the 3-port filter.
In accordance with another embodiment of the present invention, filtering voiceband and metering tone frequencies of a mixed voice/data signal involves using a lowpass filtering response to passively filter a mixed voice/data signal to pass voiceband content of the mixed voice/data signal and to suppress mixed signal content having frequencies falling within a databand of the mixed signal. Using a bandpass filtering response, the mixed voice/data signal is passively filtered to pass the metering tone frequency. The low-pass filtering response is detuned to produce a stopband between an upper end of the voiceband and the metering tone frequency. The detuned low-pass filtering response is used to suppress undesirable mixed signal content having frequencies falling within the stopband.
Passively filtering the mixed voice/data signal to pass voiceband content of the mixed signal may involve inductively and capacitively filtering the mixed voice/data signal to pass the voiceband content. Passively filtering the mixed voice/data signal to pass the metering tone frequency may involve inductively and capacitively filtering the mixed voice/data signal to pass the metering tone frequency.
A filtering methodology of the present invention may further involve using a highpass filtering response to filter the mixed voice/data signal to pass databand content of the mixed voice/data signal and to suppress mixed signal content having frequencies falling below the databand, such as mixed signal content having frequencies of less than about 30 kHz.
In accordance with another embodiment of the present invention, a circuit for filtering voiceband and metering tone frequencies of a mixed voice/data signal includes an input interface that receives the mixed voice/data signal. The input interface may receive the mixed voice/data signal from a connection having a fixed or complex impedance. A low frequency filter circuit includes an input coupled to the input interface. The low frequency filter circuit includes a lowpass filter section and a bandpass filter section of the type described above.
The filtering circuit further includes a high frequency filter circuit that has an input coupled to the input interface. The high frequency filter circuit suppresses mixed signal content having frequencies falling below the databand. An output interface of the filtering circuit includes a low frequency output coupled to an output of the lowpass filter section and a high frequency output coupled to an output of the bandpass filter section. A voiceband signal is provided at the low frequency output of the output interface and a data signal is provided at the high frequency output of the output frequency.
In one embodiment, the input interface receives the mixed voice/data signal from a POTS (Plain Old Telephone Service) connection. In this context, the filtering circuit may be implemented within or as part of a POTS splitter circuit, module or card.